The present disclosure relates generally to deposits formed on surfaces in contact with hydrocarbon fluids, and more particularly, to surfaces incorporating a texture designed to inhibit the formation of coke, soot, and oil deposits. This disclosure also relates to articles comprising such surfaces, and methods for making such articles and surfaces.
As used herein, hydrocarbon fluid is generally defined as hydrocarbon liquids, hydrocarbon gases, or mixtures thereof. As used herein, “hydrocarbon fluid degradation products” includes products which form from the hydrocarbons, for example, certain polymers resulting from thermal transformation of paraffins to cycloparaffins, aromatics and polycyclic molecules in the hydrocarbon, as well as products which result from actual decomposition of the fuel, e.g., carbon.
Because high temperature is usually associated with undesirable levels of hydrocarbon fluid deposit formation, the subject herein is commonly referred to as thermal instability, or in the case of fuels, as fuel instability. Flowing hydrocarbon fluids including lubricating oils, hydraulic oils, combustible fuels, and the like can form soot, coke, and oil deposits on the surface of containment walls and other parts which they contact, when the fluid and/or surface are heated. For example, “coking” involves the solidification of liquid fuels into carbonaceous deposits that tend to form on heated surfaces in contact with the liquid fuels. Examples of processes and systems affected by such deposition can include, petrochemical processes, machine tools, automobile engines, aircraft gas turbine engines, marine and industrial engines, and the like in which surface deposits from hydrocarbon fluids, fuels and oils are a major problem. Deposits can foul heat exchangers, plug fuel injectors, as well as lubrication distribution jets, jam control valves and cause problems with many other types of operating and control devices associated with hydrocarbon fluids, fuels and oils. Moreover, such deposition can reduce fuel flow, increase fuel line operating pressures, and reduce the performance of the injection and/or combustion system, or the entire process, system or engine.
In one example, solid deposits and varnishing occur on liquid fuel-wetted internal and external surfaces of the fuel supply system. In addition to the fuel injectors, other fuel wetted-components, including manifolds, metering valves, distribution valves, and air purge/check valves exposed to both fuel and high temperature air or high ambient temperatures can suffer from coking and carbon formation. Conditions for coking are a function of fuel composition, dissolved oxygen concentration, surface roughness, surface composition, and many other variables impact the coking rate in a hydrocarbon fuel. The current practice is to limit fuel-wetted surface temperatures to 300 degrees Fahrenheit (° F.) or less to minimize carbon formation. This is difficult in the typical gas turbine environment, where the compressor discharge temperatures for relatively low pressure ratio/performance machines exceeds 700° F., and for higher performance systems exceeds 1000° F.
A second type of carbon formation, which can extend to both liquid and gas-fired combustion systems, occurs when solid carbon particles and soot agglomerate on the combustion system components. Such carbon deposits, often called ‘clinkers’ impair air and fuel distribution in the combustor, driving up emissions, component metal temperatures, and skewing the combustor exit temperature profile, reducing the life of downstream components. Such solid carbon deposits can also cause erosion of the rotating turbine airfoils, impacting both performance and life, when these large ‘clinkers’ are dislodged by vibration, air flow, or differential thermal growth, and disintegrate in the downstream turbine. Since the collisions between the large-scale carbon deposits and airfoils can occur at very high relative velocity, (hundreds or even thousands of ft/s), and the carbon is remarkably hard in this form, erosion of the airfoil surfaces is a durability problem.
One particular problem area for liquid fueled on-wing and aeroderivative engines is the splashplate. On these types of engines, coke can build up on the splashplate and eventually the coke can flake off and damage barrier coatings on the combustor components. Such problems can have a severe impact on the operability of the engine. Another area of concern with coking in current gas turbine engines are the fuel lines leading up to the combustor. If the fuel line temperature is within a certain temperature window, coke can form inside the fuel lines, increasing the required pumping pressure and/or restricting fuel flow to the engine.
As mentioned above, one method of mitigating the build-up of coke and other deposits in fuel lines and other contact surfaces of turbine engines has been to keep the temperature low enough to prevent the coke deposition reaction from occurring. Such turbine engines, however, are forced to operate at less than optimal temperatures, and therefore, can be inefficient. Alternatively, cooling apparatuses have been added to the combustion systems to keep the temperatures of the surfaces lower without sacrificing optimal ignition temperatures. The apparatuses, however, add cost and complexity to the turbine engine designs. Another method has been to coat the surfaces with a catalyst or coating, sometimes referred to as Coke Barrier Coatings (CBC), wherein the coatings are chemically designed to inhibit the bonding of thermal deposits to the surfaces. Again, the special coatings add cost and an additional process step to the design of a turbine engine. Moreover, some current coatings are not suitable for every type of thermal deposition that can occur in the combustion system. Yet another method of dealing with thermal deposition has been to modify the hydrocarbon fuel with additives. The fuel must be pre-treated before being used, however, or special fuels with the additives already mixed must be purchased at a premium.
A surface for a metal article which inhibits the formation of thermal deposits, particularly an anti-coking surface, without resorting to modification of the hydrocarbon fluid, without adoption of special procedures, and without the installation of special equipment could be desirable for current liquid fuel turbines and other like apparatuses.